Many aircraft are equipped with retractable landing gears, which typically can be raised following take-off and stowed in a closable compartment on the underside of the aircraft, e.g. in the wings. A landing gear may often be provided under either wing and under the nose of the aircraft.
When the landing gears have been raised, the gears are locked in place with uplocks, to prevent unintended extension of the landing gears, until such time as the landing gears are to be used again. It is desirable to include safety measures to ensure that when the landing gears are extended they do not accidentally retract, and that in the event of a malfunction of the hydraulic system, the landing gears can always be extended as necessary for landing.
A known hydraulic landing gear system 1 is shown schematically in FIG. 1 of the drawings, wherein a system including three landing gears is shown. A hydraulic fluid pressure line 5 and a hydraulic fluid return line 6 are connected to a hydraulic source (not shown) which is usually provided as part of the aircraft hydraulics system. A selector valve 2 is provided along the pressure and return lines 5,6 which is operable to direct the hydraulic pressure along first and second conduits 17,18 which lead to a plurality of hydraulic actuators 8,9,10,11,12,13. The actuators numbered 8,9 and 10 comprise first, second and third gear downlock actuators respectively, which are operable to lock the landing gears in the extended position, whilst the actuators numbered 11, 12 and 13 comprise first, second and third gear actuators, for extending and retracting the landing gears, and optionally the doors of the landing gear compartments. Alternatively, the doors may be actuated by means of a separate hydraulic system.
An uplock control valve 4 is hydraulically connected to the first conduit 17 for controlling first, second and third uplocks 14,15,16. The uplocks lock the landing gears in the retracted position to prevent unintended extension of the landing gears. A corresponding uplock system can be provided to control opening and closing of the doors to the closable compartments in which the landing gears are housed.
In one example, the first uplock 14, the first downlock actuator 8 and the first gear actuator 11 correspond to the right wing landing gear, while the second uplock 15, the second downlock actuator 9 and the second gear actuator 12 correspond to the left wing landing gear and the third uplock 16, the third downlock actuator 10 and the third gear actuator 13 correspond to the nose landing gear.
The selector valve 2 comprises first and second solenoids 19, 20 for moving the valve between first, second and third settings 23,24,25. First and second springs 21, 22 centre the valve, whereby the second setting 24 is the default status of the valve. When it is desired to extend the landing gears, the first solenoid 19 is operated to bias the valve elements to the right, thereby putting the valve in the first setting 23. Pressure is thus applied to the actuators 8,9,10,11,12,13 so as to extend the landing gears. In order to retract the landing gears, the second solenoid 20 is operated to bias the valve elements to the left, thus placing the valve in the third setting 25. When neither solenoid 19,20 is operated, the second setting 24 is selected. In the second setting 24, the valve inhibits the pressure from the pressure line 5 and connects both conduits 17,18 to the return line 6, thereby removing hydraulic pressure from the system.
The landing gear system also comprises a fail-safe free-fall system including a cut out valve 3 and a vent valve 7, which in the event of a selector valve 2 failure, are operable to ensure that the landing gears can be extended. In free-fall operation, the cut out valve 3 isolates the system from the hydraulic pressure, whilst the vent valve 7 provides an interconnection between the fluid conduits 26,27 downstream thereof. This results in a status of the system where the landing gears can extend under the influence of gravity.
The prior art hydraulic landing gear system 1 is complicated and backup systems are needed to operate the cut out and vent valves. Further, it is possible for inadvertent retractions of the landing gears to take place if a fault occurs in the selector valve 2. For example, if the second solenoid 20 is erroneously operated, e.g. due to a malfunction, the selector valve will move so that the valve element 25 is operative and the landing gears will retract. When the aircraft is on the ground, such a fault is both expensive and dangerous. Complex control and backup systems have to be provided to cater for the potential erroneous operation or failure of the solenoids 19,20. The prior art system is therefore expensive and is heavy due to the relatively large number of components needed.